I. Field of the Invention
This invention relates to a lubrication system for the inner chambers of a drilling head incorporating at least one rotating stripper rubber and, in particular, to a system for maintaining optimum pressure balance within the chambers relative to the wellbore to prevent contaminants from entering the chamber while also supplying lubricant across the seals for controlled operation of the seals and increased seal life.
II. Description of the Prior Art
With the increased use of rotating drilling heads which incorporate at least one stripper rubber that rotates with the kelly drive and therefore the drill string, a bearing assembly is needed to rotatably support the drive assembly. The stripper rubber is mounted to the drive ring and drive bushing which rotate within the drilling head. Disposed within an annular chamber between the drive ring and the outer wall of the drilling head housing is the bearing assembly. The bearing chamber is sealed at both ends to prevent contamination of the lubricant circulating through the chamber. Typical drilling heads also include a seal assembly associated with the drive ring which can fail if contaminated. These seals are associated with a secondary chamber formed by a series of fluid passageways. The bearing chamber and the seal chamber may or may not be in communication with each other. Lubricant may be continuously supplied to these chambers or a self-contained lubricant supply may be utilized. In any event, the purity and volume of the lubricant depends upon the integrity of the seals particularly those at the lower end of the chamber which are subject to the extreme pressures of the wellbore. The operating life of the drilling head can be dependent upon the integrity of the seals in order to prevent more costly damage.
In the prior known drilling heads, a substantial increase in wellbore pressure would force well fluids past the seals introducing contaminants to the chambers. Conversely, a sudden drop in wellbore pressure results in an increase in the pressure differential forcing lubricant out of the chamber past the seals. In a chamber with a fixed supply of lubricant this can reduce the operating life of the drilling head. Additionally, the integrity of the seals is compromised which can result in their failure as the wellbore pressure increases. Some prior known systems have attempted to electrically compensate for variations in pressure using electronic sensors and valves. However, such a system can be cost prohibitive and subject to failure in the grimy environment of a drilling head.